The above-named professional personnel propose to collaborate with other members of the Children's Cancer Study Group as well as with other members of other cooperative cancer study groups, in research on leukemia, malignant solid tumors and closely related disorders in children. The chief purpose of this study group is to cooperate in research on malignant diseases in children and, in particular, to study factors which may cause or influence cancer and to develop, evaluate, and improve methods of diagnosis and treatment of malignant diseases in children. Special emphasis will be placed on the study of new combinations and new schedules of already established chemotherapeutic agents as well as on the evaluation of new anti-cancer agents and new modalities of therapy. A particular interest of the Principal Investigator is Histiocytosis X. A new protocol has been designed to study this disorder in depth with particular attention to clinical and histologic features of prognostic significance, to improved therapy and to comprehensive immunologic evaluation.